Air strikes have hit the only road into rebel-held areas of Aleppo city in the heaviest bombing since February, jeopardising access where around 300,000 Syrians live.
Russian warplanes carried out the attacks on the Castello road, which is still open but dangerous, a rebel official and monitors say.
Defence officials from Syria's government and its ally Russia could not immediately be reached for comment.
The Syrian Observatory for Human Rights group said the road was hit in a week of escalating air strikes, with Sunday's attack the most intense yet.
The city of Aleppo, about 50km south of the Turkish border, is divided between the government and rebels fighting to topple Syrian President Bashar al-Assad.
A truce was brokered by the US and Russia in February. But the agreement has since unravelled, with fighting and bombardment in Aleppo playing a big part in its collapse.
Kurdish-led YPG forces, which control the Sheikh Maqsoud area in Aleppo that overlooks the Castello road and are tacitly aligned with the government, have also disrupted the road with snipers who target civilians using the road that is a lifeline for the city to the countryside.
Meanwhile mainstream Syrian rebel groups said they would no longer abide by the UN truce deal unless the Syrian army ended a major assault on their positions in the suburbs of Damascus within 48 hours.
A statement by the Free Syrian Army signed by nearly 40 rebel groups that operate across Syria said they would deem the cessation of hostilities deal as having "totally collapsed" if the assault by Syrian government and allied Lebanese Hezbollah forces fighters did not cease within two days.
The signatories, who include Western- and Turkish-backed groups operating on the main front lines in northern and southern Syria, said that once the two-day period had ended, rebels would respond with "all the legitimate means to defend the civilians living in these areas".
In their joint statement, the rebels said the continued attacks by the army on the besieged rebel-held areas around Damascus and their strongholds in Aleppo city and Idlib province were putting peace-making efforts at risk.
A senior official in the rebel group Fastaqim that operates in the Aleppo area said there was heavy Russian bombing of rebel areas on the strategic route all day, developments the Syrian Observatory confirmed later.
"From 1am until 10am, Russian jets were not quiet on the Handarat-Castello front," said Zakaria Malahefji. "A group [of fighters] stationed there was killed."
Observatory director Rami Abdulrahman said the raids had been happening for a week. "This is more intense than the last days," he added.
Heavy air strikes have hit the rebel road to Syria's Aleppo, while rebels have threatened to end a truce if the regime doesn't stop attacks in Damascus.
A Russian defence ministry statement issued on Saturday accused insurgents of firing missiles at nearby areas, identifying them as members of the al Qaeda-linked Nusra Front, which was not included in the truce.
Rebels fighting under the banner of the Free Syrian Army say Nusra has little or no presence in Aleppo city.